The antenna of the invention has applications in particular in the context of mobile satellite communication between stationary users and/or mobile telephones of any type, for example, aeronautical, seaborne or terrestrial. In this field, several satellite communication systems are implemented, or are currently being developed (for example, INMARSAT, INMARSAT-M, GLOBALSTAR systems (registered trademarks), and so on). These antennas are also useful in the deployment of personal communication systems (PCS) by geostationary satellites.
The objective of these systems is to provide terrestrial users with new communications services (multimedia, telephone) via the satellites. Using geostationary or moving satellites, they enable global terrestrial coverage to be obtained. They must be similar to terrestrial cellular systems in terms of cost, performance and size. Thus, the antenna located on the user's terminal is a key element from the point of view of size reduction.
Such systems are described in particular in the documents of Howard Feldman, D. V. Ramana: “An introduction to Inmarsat's new mobile multimedia service”, Sixth International Mobile Satellite Conference, Ottawa, June 1999, and J. V. Evans: “Satellite systems for personal communications”, IEEE A-P Magazine, Vol. 39, no. 3, June 1997.
For all of these systems, which provide connections with geostationary satellites, the very different incidences of the signals received or transmitted require antennas to have an hemispherical or near-hemispherical coverage radiation pattern. In addition, the polarisation must be circular (left or right) with a ratio below 5 dB in the useful band.
More generally, the invention may have applications in all systems requiring the use of a broadband and a circular polarisation.
In these various fields of application, the antennas must indeed have the aforementioned features either in a very broad band of approximately 10% or more, or in two adjacent sub-bands corresponding to the reception and the transmission, respectively.
A type of quadrifilar helical antenna particularly suitable for such applications is already known from patent document FR-8914952 of France Telecom (registered trademark). A quadrifilar antenna is made of four radiating wires.
This antenna, referred to as a printed quadrifilar helical (PQH) antenna, has features similar to those mentioned, in a frequency band limited in general to 6 or 8% for an SWR below two.
A broader band operation can be obtained using two-layer PQH antennas. These antennas are formed by the concentric “interleaving” of two coaxial quadrifilar resonant helices which are electromagnetically coupled. The assembly functions as two coupled resonant circuits, of which the coupling deflects the resonance frequencies. Thus, a two-layer quadrifilar resonant helical antenna is obtained according to the technique described in FR-8914952.
This technique has the advantage of requiring a single power supply system, and of enabling double- or broadband operation.
However, it has the disadvantage of requiring two printed and overlapping circuits to be produced, and, in the double-band operation, of providing only a narrow band in each sub-band. In broadband operation, the bandwidth obtained remains limited.
Another embodiment is described in detail in the document “Analysis of quadrifilar resonant helical antenna for mobile communications”, by A. Sharaiha and C. Terret (IEE—Proceedings H, vol. 140, no. 4, August 1993).
According to this embodiment, the radiating wires are printed on a thin dielectric substrate, then wound around a transparent cylindrical support with radioelectric transparency. The four helical wires are opened or short-circuited at one end, and electrically connected at the other end.
This antenna requires a power supply circuit that provides the excitation of the different antenna wires by signals of the same amplitude in phase quadrature. This function can be achieved using 3 dB −90° coupling structures and a hybrid ring. The assembly can be placed in a printed circuit and positioned at the antenna base. A simple yet bulky power supply is thus obtained.
As mentioned above, it is desirable for the antenna (including its power supply) to be of the smallest possible size and lowest possible weight, and to have the lowest possible cost.
Several approaches which seek to reduce the size of the antenna and its power supply system have been proposed. In particular, as examples, the following solutions can be cited:                in patent document FR-9603698, of France Telecom (helical antenna with integrated broadband power supply);        in patent document FR-0011830, of France Telecom (helical antenna with adjustable pitch);        in patent document FR-0011843, of France Telecom (helical antenna with wires of adjustable width); and        in the article of B. Desplanches, A. Sharaiha and C. Terret entitled “Parametrical study of printed quadrifilar helical antennas with central dielectric rods” (Microwave and Opt. Technol. Letters, vol. 20, no. 4, Feb. 20, 1999).        
Nevertheless, these antennas do not have a very large bandwidth.
The prior art also describes helical antennas with bent radiating elements shown respectively in a patent document U.S. Pat. No. 6,229,499 of the XM Satellite Radio company (registered trademark) and in a patent document U.S. Pat. No. 6,278,414 of the Qualcomm company (registered trademark). These antennas have radiating elements which are partially bent onto themselves thus enabling their height to be reduced. Nevertheless, these antennas have the disadvantage of having a narrow bandwidth.
The aim of the invention is in particular to overcome these various disadvantages of the prior art.